Conventionally, if no overcurrent protection function or circuit corresponding thereto is separately provided, switching power supply circuits exhibit different characteristics in an overcurrent condition and in the event of a load short-circuit, depending on the operational topology of the switching power supply circuits.
For example, flyback switching power supply circuits exhibit a gentle decrease of an output voltage with an increase of output current in the event of an overcurrent. This is generally known as “inverted V-shaped” characteristics (constant-power control voltage droop type). Forward switching power supply circuits exhibit a decrease of an output voltage and an output current, upon an increase of the output current in the event of overcurrent. This is generally known as droop characteristics.
When a switching power supply circuit has the characteristics as described above, a semiconductor device is overloaded in the event of overcurrent and may be damaged due to abnormal overheat. For this reason, a protection circuit may be added for overcurrent protection and load short-circuit protection, in addition to the basic circuitry for the switching power supply (a component configuration necessary as a topology).
There is also a method which prevents abnormal overheat of the components of the switching power supply circuit, in the events of load short-circuit and overcurrent, without adding such a protection circuit. The method prevents the abnormal overheat by intermittent operation which makes use of a decrease of a bias voltage with a decrease of the output voltage.
Upon load short-circuit condition or overcurrent condition in the switching power supply circuit, electrical energy conveyed by a transformer temporarily rapidly increases, and a voltage generated at an auxiliary winding may increase greater than an original design value. However, most of the energy is absorbed into a capacitor included in the auxiliary winding and thus the bias voltage is less likely to jump up. Then, due to the action, by a circuit, of sensing the load short-circuit condition or overcurrent condition, a ratio of on-time of a primary-side switch circuit to a switching cycle (Hereinafter, referred to as ON-Duty or a duty ratio of a pulse width modulation (PWM) signal) decreases, which decreases a secondary-side output voltage and the bias voltage that is obtained from the auxiliary winding of the transformer (Operation 1).
As the bias voltage decreases to a stop voltage for a power supply control IC, the power supply control IC causes the primary-side switch circuit to stop operating. A capacitor connected to the power supply terminal of the power supply control IC is then charged with a voltage input via an activation resistor. As the capacitor has reached a start voltage for the power supply control IC, the power supply control IC causes the primary-side switch circuit to operate again (Operation 2).
If the load short-circuit condition or overcurrent condition persists, the above operations 1 and 2 are repeated, thereby preventing the switching power supply circuit and the load connected thereto from being abnormally heated.
The repeating of the operations 1 and 2 as the above is known as an intermittent operation. The intermittent operation is a common as a protection operation to take upon the load short-circuit condition or overcurrent condition. Such an intermittent operation can prevent the switching power supply circuit, in the load short-circuit condition or overcurrent condition, from being abnormally heated, thereby improving the safety of the switching power supply circuit.
Moreover, in the switching power supply circuit, for the power supply terminal of the power supply control IC which outputs a signal that drives the primary-side switch circuit, an applied voltage at which the primary-side switch circuit initiates the switching operation (e.g., start voltage 16V), a voltage at which the primary-side switch circuit stops the switching operation (e.g., stop voltage of 10 V), and a maximum applied voltage (e.g., 30 V) for preventing the breakdown of the power supply control IC are defined. Thus, the switching power supply circuit includes a constant voltage circuit (a transistor, a resistor, a diode, a constant voltage diode, etc.), for the purpose of preventing the breakdown of the power supply control IC, so that the bias voltage output from the auxiliary winding of the transformer is less than or equal to a constant voltage, (e.g., see Japanese Patent Laying-Open No. 2015-173043 (PTD 1), Japanese Patent Laying-Open No. H7-213053 (PTD 2)).